1. Field of the Invention
The present invention relates to the field of power supply circuits, low dropout regulators, linear regulators, pass elements and the like wherein protection of electrical components is required when the output voltage is greater than the input voltage.
2. Prior Art
Although there are many MOS and CMOS linear voltage regulators currently available in monolithic form, none of these regulators is protected against the situation where the output voltage can be greater than the input voltage. Although there are many applications where this is not a concern, new multiple powered systems using batteries and line operated power sources require that the system voltage(s) be maintained under discharge, failure or turn-off of one or possibly more of the input power supplies. This means that if an input power supply is connected to a system through a linear regulator having a MOS pass element, reverse current could flow through the pass element from the output of the linear regulator to the input power supply. At the least, this is undesirable since it wastes power and at the worst, if the reverse current is high, component or system damage may result. Although this discussion will use the linear regulator as an example, the invention is not limited to linear regulators since there are other regulator topologies such as buck switching regulators that could benefit from the invention.
To date the most popular monolithic linear voltage regulators have been fabricated using bipolar semiconductor technologies where the reverse characteristics (reverse current gain etc.) of a pass transistor are generally much inferior to the forward characteristics. Consequently, for a given base current, the reverse collector current would be a couple of orders of magnitude less than the forward collector current. In addition, most bipolar regulators detect when the pass transistor goes into saturation and automatically reduce the base drive current. The result of this is that, although reverse current may flow, it is often of a relatively low value and, for most applications, is not a problem.